A secondary battery has high applicability depending on the product group and excellent electrical characteristics such as high energy density, and thus is commonly used as an electric power source of electric vehicles (EV) or hybrid vehicles (HV) as well as mobile devices.
Such a secondary battery significantly reduces the use of fossil fuels and does not generate by-products that come with energy consumption, which can improve environmental friendliness and energy efficiency. For these reasons, a secondary battery is gaining attention as an alternative energy source.
Recently, with the increasing significance of global issues relating to the exhaustion of energy resources such as fossil fuels, environmental pollution, economically efficient energy use, and the like, a smart grid system has been actively studied to effectively overcome the inequality in power consumption and power production and solve the problems caused thereby, such as, for example, the waste of power when power is oversupplied and power overload when power supply is in shortage. The smart grid system flexibly controls the power supply using a variety of information and communication infrastructures. These smart grid systems are being vigorously studied and such a secondary battery is gaining attention as a source for an efficient energy storage and application in the smart grid system.
When a secondary battery is used as a battery for instruments such as a mobile terminal, although not necessarily so, it is conventionally used in the form of an assembly of a plurality of secondary unit cells to satisfy a high capacity need.
As schematically shown in FIG. 1, in the case that a battery pack 10 consisting of an assembly of a plurality of secondary cells 1 is used in electric vehicles, a battery managing apparatus 30 such as a BMS (battery management system), which monitors and controls the status of the secondary cells by means of a power supply control for driving loads such as a motor, the measurement of electrical characteristics such as current, voltage and the like, the control of charging and discharging, the control of voltage equalization, the application of an algorithm for estimating a SOC (state of charge) may further be included.
The battery managing apparatus 30 controls an electronic control device 31 and a power system provided in a vehicle system 20 consisting of driving loads such as a motor, as mentioned above, and also transmits information on the present SOC or SOH (state of health) of the battery pack 10 into the vehicle system 20, to interface the information on the present status of the battery to a user or a driver.
As mentioned above, the battery pack 10 used in vehicles consists of a plurality of secondary cells connected in series and/or parallel, and the battery pack supplies power in loads such as a motor, a generator, an electric installation through a system prepared by the electric connection of the plurality of secondary cells.
In this regard, a power system having secondary cells may not provide state information on a presently usable power in a physical and absolute criterion, unlike a gasoline engine system, and so various methods estimating or predicting SOC by using various electrical characteristics or parameter information are known in the art.
Meanwhile, a plurality of secondary cells constituting the battery pack generates a capacity difference between the cells, which results from the intrinsic characteristics, differences in manufacturing environment, diversity of system application as use time passes, and further leads to the terminal voltage difference or SOC difference of a relevant secondary cell due to charging and discharging.
Such a difference can result from coulomb efficiency, which is generated as each cell has a different dynamic state separate from each other, or by a differential variation of an internal resistance according to a physical configuration. Thus, in the case that the plurality of secondary cells having relatively different electrical characteristics operates in a single battery pack, there is a problem that the charging or discharging capacity of the whole battery pack is restricted by a certain secondary cell having a degraded performance.
According to the fundamental characteristics, a plurality of secondary cells constitutes a battery pack having different profiles for each cell, and thus whether a secondary cell is degraded or not and the degree of degradation for each cell are individually different. Nevertheless, the conventional SOC estimating methods have been made simply by only using statistical values such as an average voltage value of the secondary cells, regardless of the fundamental characteristics and statuses of the secondary cells, so the conventional methods do not provide the present performance of the battery pack or substantial and efficient information on the SOC thereof. Particularly, secondary cells used for a long period of time have even greater electric characteristic differences.
The stability and safety of electric vehicles for which the secondary cells are used is a significantly important issue, and a problem regarding thereto may cause unpredicted errors during the operation or application of the vehicles and, in a severe case, stop the operation of the vehicles. This is an important problem which may not be solved by simply applying numerical values obtained from the calculation of the SOC.
Also, the conventional methods for balancing or equalizing a performance difference for each secondary cell include a bucking method of consuming the voltage and charged energy of the secondary cells having electrical characteristic values (SOC, voltage or the like) not lower than a certain reference value, to equalize the electrical characteristic values of the secondary cells, or shuffling methods including a boosting method of maintaining a selective interruption to the outside and further conducting a separate charging for only secondary cell having a lower electrical characteristic value.
However, such equalization methods are conducted in the form of integrated processing for all secondary cells included in a battery pack without taking into consideration the selection of a certain cell by considering respectively a degradation state of each secondary cell, and thus the methods are unfavorable in terms of effectiveness.